17 Inch Rims versus 18's, and other concerns?

My local Ford dealer just got in a car that is almost the exact car that I want, except for the Roof color, Mach 500, and the 17 inch rims.

For those who forgot, I desire a Parchment over Tungsten GT Premium with 18's (Non Bullits) with the shaker 1000.

I know I may be able to get a better deal on a car sitting on the dealers lot than one I order, especially in February in Michigan, since the car will likely have a longer shelf life here than in a Southern State. I just wonder if it would be worth the sacrifice. I am now torn on what color roof I want, b/c the black roof on the car looks sharp. I am more concerned about the rims and the shaker 500. I think I would like the bigger rims and for that matter the "bigger" audio.

What are your thoughts, is the 500 adequate, and what about the smaller rims? they are the 17 inch non bullits. So, they sort of have the look I desire.

Thanks!